# perf

**perf** is a Linux profiling tool that reads hardware performance counters from the CPU. Measure cycles, instructions, cache misses, branches—everything the CPU tracks natively. Answer the fundamental question: where is the program spending time, and why is it slow?

Unlike guessing, perf gives ground truth from the hardware. Low overhead (1-5%), no recompilation needed, and the data is reliable. In HPC work, perf is the first tool to reach for when something is slower than expected.

```bash
$ perf stat ./program
Performance counter stats for './program':
     4,102.38 msec task-clock              # 3.98 CPUs utilized
    16,847,203,412 cycles                  # 4.107 GHz
    12,334,901,088 instructions            # 0.73 insn per cycle
       401,292,771 cache-misses            # 33.33% of cache refs
```

IPC (instructions per cycle) of 0.73 on a modern superscalar CPU means the pipeline is stalled. A 33% cache miss rate means it's waiting for memory. Two numbers tell you everything you need to know to start optimizing.

## Concepts

 1. [[perf-basics|Basics]]
 2. [[perf-stat|perf stat]]
 3. [[perf-record|perf record]]
 4. [[perf-report|perf report]]
 5. [[perf-annotate|perf annotate]]
 6. [[perf-events|Events]]
 7. [[perf-metrics|Metrics]]
 8. [[perf-flame-graphs|Flame graphs]]
 9. [[perf-sampling|Sampling]]
 10. [[perf-scripting|Scripting]]